A frequency multiplier (e.g. a tripler) is a device that, as an example, takes an input signal at 30 GHz and converts some of the energy from this input signal into an output signal at 90 GHz. Because a frequency multiplier is used to generate an output at a different frequency from the input, one of the goals of an optimized structure is efficient operation whereby the energy transferred to the new frequency at the output is maximized. The same methodology for the device/structure and optimization can also be applied to obtain higher harmonics, such as the fifth order of conversion, which will be useful for sub-millimeter wave signal generation.
Varactor multipliers have first been conceptualized back in the mid 1960s. However, little has been done insofar as a conceptual method on how to optimize the device-material or structure for highest frequency conversion efficiency at a particular output power level.
According to Wikipedia, varactors are operated in a reverse-biased state. No current flows, but since the thickness of the depletion zone varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region thickness is proportional to the square root of the applied voltage; capacitance is inversely proportional to the depletion region thickness. Thus, the capacitance is inversely proportional to the square root of applied voltage. The depletion layer can also be made of a MOS or a Schottky diode.